An egg is a reproductive structure produced by female birds, containing the nutrients needed for embryo development. While many eggs are food, some possess the potential to develop into a new life. Understanding the difference between a fertile and an infertile egg is a common inquiry, particularly for those interested in poultry. This distinction lies in whether the egg has been fertilized and can therefore potentially grow into a chick.
What Makes an Egg Fertile
A fertile egg contains a fertilized ovum, with genetic material from both a male and female bird. This fertilized ovum, called a blastoderm, can develop into an embryo if incubated properly. In contrast, an infertile egg is an unfertilized ovum, or blastodisc, which will never develop into a chick.
When examining the yolk, a fertile egg displays a blastoderm that looks like a bullseye or a larger white spot with a clear center. An infertile egg, however, has only a smaller, irregular white spot without a distinct ring pattern.
The Process of Fertilization
For an egg to become fertile, a rooster must mate with a hen. This process, known as cloacal contact, involves the rooster transferring sperm to the hen’s reproductive tract. Unlike mammals, roosters do not have a penis; their cloacas (the common opening for waste and reproduction) touch, allowing sperm transfer.
Once inside the hen, sperm travels to the oviduct, where it can be stored for up to three weeks. Fertilization occurs internally within the hen’s oviduct, specifically in the infundibulum, before the egg’s shell forms. If sperm is present when the ovum is released, it can fertilize the egg. The fertilized egg then continues its journey through the oviduct, acquiring albumen and shell layers before being laid.
Identifying Fertile Eggs
Distinguishing a fertile egg from an infertile one visually from the outside is not possible, as they look identical. The most common method to identify fertility without breaking the egg is called candling. Candling involves shining a bright light through the egg in a darkened room, illuminating its interior.
When candling a fresh fertile egg, a small, faint “bullseye” pattern on the yolk indicates the blastoderm. After a few days of incubation, a fertile egg will show a visible network of spider-like blood vessels radiating from a central dark spot, which is the developing embryo. An infertile egg, when candled, appears uniformly clear or translucent, lacking any internal structures or blood vessel development.
Consuming Fertile Eggs
Fertile eggs are safe and nutritious for consumption. There is no difference in taste or nutritional value between fertile and infertile eggs that have not been incubated. Most eggs sold commercially are infertile because laying hens in large-scale operations typically do not have access to roosters. If fresh fertile eggs are collected daily and refrigerated, embryonic development is halted, ensuring they remain suitable for culinary use.